2.8 Sculpting Resolution?
The size used by dynamic topology relates to the visible size of the mesh details. You can also think of it as distance between vertices.

So the smaller the size used, the smaller the details, which means more vertices in your mesh that are closer together.

What the size value refers to depends on the detail type selected :-


Relative Detail uses screen pixels
Constant Detail uses blender units
Brush Detail uses the brush size